How to add text below a vertical line in a Gantt chart

Question

Is it possible to add text below the x-axis in a Gantt chart. For example, assume I have a simple Gantt chart with two tasks

\begin{ganttchart}[vgrid, hgrid]{12}
\gantttitle{Title}{12} \\
\ganttbar{Task 1}{0}{3} \\
\ganttbar{Task 2}{3}{10} 
\end{ganttchart}

I'd like to add text below some of the vertical grid lines in the Gantt chart. For example, below the vertical grid line where Task 1 ends I'd like to add the text "T_1" and add "T_2" below the vertical grid line where Task 2 starts. Is this possible?

Answer 1

Here's one way to do it using a variation of Paul Gessler's nice answer to How to draw a vertical line at a specified date in a Gantt chart?:

\documentclass{article}
\usepackage{pgfgantt}

\makeatletter
\@gtt@keydef{anyday}{none}
\@gtt@keydef{anyday offset}{1}
\@gtt@stylekeydef{anyday rule}{dashed, line width=1pt}
\@gtt@keydef{anyday label font}{\normalfont}
\@gtt@stylekeydef{anyday label node}{%
  anchor=north, font=\ganttvalueof{anyday label font}%
}

\newcount\gtt@anyday@slot

\newcommand\gantText[2]{%
  \gtt@tsstojulian{#1}{\gtt@anyday@slot}%
  \gtt@juliantotimeslot{\gtt@anyday@slot}{\gtt@anyday@slot}%
  \pgfmathsetmacro\y@upper{%
    \gtt@lasttitleline * \ganttvalueof{y unit title}%
  }%
  \pgfmathsetmacro\y@lower{%
    \gtt@lasttitleline * \ganttvalueof{y unit title}%
      + (\gtt@currentline - \gtt@lasttitleline - 1)%
      * \ganttvalueof{y unit chart}%
  }%
  \pgfmathsetmacro\x@mid{%
    (\gtt@anyday@slot - 1 + \ganttvalueof{anyday offset})%
      * \ganttvalueof{x unit}%
  }%
  \node [/pgfgantt/anyday label node]
     at (\x@mid pt, \y@lower pt) {#2};
}
\makeatother

\begin{document}

\begin{ganttchart}[vgrid, hgrid]{1}{12}
\gantttitle{Title}{12} \\
\ganttbar{Task 1}{1}{3} \\
\ganttbar{Task 2}{3}{12}  \\
\gantText{3}{$T_1$}
\gantText{2}{$T_2$}
\end{ganttchart}

\end{document}